at the united nations today, israel's prime minister, benjamin netanyahu, wander that iran's work to build a nuclear bomb will be irreversible by next summer. that is significant because he's never put a time frame like that on it before. >> and i believe that faced with a clear red line, iran will back down. and this will give more time for sanctions and diplomacy to convince iran to dismantle its nuclear weapons program altogether. >> pelley: now, iran's president, mahmoud ahmadinejad, denies that iran is building a bomb. so we asked david martin to tell us more. >> reporter: it looked like a cartoon, but it was deadly serious. israel's prime minister netanyahu drew the red line that would trigger an attack on iran's nuclear program as clearly as it has ever been drawn. >> a red line should be drawn right here, before-- before iran completes the second stage of nuclear enrichment necessary to make a bomb. >> reporter: that second stage is taking place at this underground facility near the holy city of cum, where iran is enriching your anium to a 20% level of purity, one step away f